This invention relates to the art of papermaking. More particularly, it relates to the art of multi-ply papermaking which is sometimes referred to as boardmaking. Still more particularly, this invention relates to the art of producing separate plies of paper web individually with a plurality of forming wires and bringing the separately produced web plies together to effect ply bonding into a composite board. The resultant product is then removed from the forming section of the papermaking machine and transferred to pressing, drying and winding stations for further processing.
Typical examples of prior multi-ply web forming apparatus are shown and described in U.S. Pat. Nos. 3,681,193 (Nykopp), 3,726,758 (Parker et al) and 3,954,554 (Curry et al). Typically, prior multi-ply web forming apparatus either form the top ply with minimum dewatering through its lower side (Parker et al), or dewater the top ply only through one side (the lower side) of a horizontally-extending forming unit (Curry et al), or quickly form the top ply over a roll former with the side to be bonded to the base ply dewatered significantly more than the other side (Nykopp). Each of these structures has disadvantages which result in either a composite multi-ply web being formed more slowly than economically permissible in today's competitive environment, or wetter. Conversely, the composite web product might be formed at a competitive speed, but the resulting product would be either lighter in weight than desired or possess insufficient ply bonding to provide a saleable product, or both.
Such ply bonding deficiencies can be generally attributed to decreased affinity between webs which have lower fines content in their mating faces. Also, especially where either or both of the mating plies contain an abundance of fines, which promote ply bonding, excessive speed in the rate of water removal through the faces of the plies to be mated adversely affects ply bonding because it tends to draw a greater amount of fillers to the surface. Fillers do not enhance ply bonding.